1. (Field of the Invention)
The present invention generally relates to a display device utilizing a shadowmask color cathode ray tube and, more particularly, to a scanning device for driving electron beams in the shadowmask color cathode ray tube.
2. (Description of the Prior Art)
As shown in FIG. 12, the conventional shadowmask color cathode ray tube utilizing a finely perforated shadow mask comprises a highly evacuated envelope 12 having a longitudinal axis Z and including a funnel section 13, a neck section 14 continued from one end of the funnel section 13 and a faceplate 15 sealed to the other end of the funnel section 13. The faceplate 15 has a phosphor deposited screen 16 having a predetermined pattern of triads of luminescent phosphor dots deposited thereon, and the neck section 14 accommodates therein an electron gun assembly 17 from which electron beams corresponding in color to the three primary colors, for example, red, green and blue, towards the phosphor deposited screen 16. The shadow mask 18 has a multiple of minute apertures or slots 19 defined therein in a predetermined regular pattern related systematically to the pattern of triads of the luminescent phosphor dots on the phosphor deposited screen 16 and allows the passages of the electron beams therethrough before the electron beams impinge upon the phosphor deposited screen 16. In FIG. 12, reference numeral 30 represents a deflection yoke mounted on the evacuated envelope 12, and reference characters X and y represent axes perpendicular to the longitudinal axis Z, while X-axis and Y-axis lie parallel to horizontal and vertical directions, respectively, on the phosphor deposited screen 16.
It has often been observed that, since the electron beams emitted from the electron gun assembly form substantially equally spaced scanning lines when sweeping across the phosphor deposited screen, shades delimited by the regularly patterned minute apertures or slots in the perforated shadow mask and the similarly regularly spaced scanning lines interfere with each other resulting in the appearance of undesirable Moire patterns on the screen of the color cathode ray tube.
A major cause of the appearance of the undesirable Moire patterns on the screen of the conventional display device utilizing the shadowmask color cathode ray tube will now be discussed in detail with particular reference to FIG. 5 of the accompanying drawing.
FIG. 5 (a) illustrates a portion of the phosphor deposited screen 16 showing a mosaic pattern of the luminescent phosphor dots in the conventional shadowmask color cathode ray tube of the type utilizing the finely slotted shadow mask 18 (FIG. 12) and FIG. 5 (b) illustrates a graph showing a change in average luminous efficiency relative to the mosaic pattern shown in FIG. 5 (a). The luminescent phosphor dots generally identified by 21 is of a substantially rectangular shape similar to the substantially rectangular shape of each minute aperture 19 in the perforated shadow mask 18 (FIG. 12) and are regularly distributed over the screen in a direction parallel to the Y-axis, each neighboring luminescent phosphor dots 21 being spaced apart from each other by the presence of a non-luminescent bridge portion 20.
The mosaic pattern of luminescent phosphor dots on the screen is composed of a multiple of equally spaced, parallel rows of the luminescent phosphor dots 21 each row extending parallel to the Y-axis, while the luminescent phosphor dots 21 of one row are offset relative to the luminescent phosphor dots 21 of the next adjacent row by half a pitch of the dots 21 in a direction parallel to the Y-axis.
Since the luminescent phosphor dots 21 of each row alternate with the non-luminescent bridge portions 20 of a corresponding row, the total radiation of light produced by the luminescent phosphor dots 21 of any two adjacent rows in a direction perpendicular to the Y-axis as a result of electron bombardment may be described in terms of an average luminous efficiency in the Y-axis direction. If the average luminous efficiency is expressed by TA (Y), the average luminous efficiency depicts such a curve as shown in FIG. 5 (b) and can be expressed by the following equation when transformed according to Fourier's series. ##EQU1## wherein PA represents the cycle (pitch) of this function in the Y-axis direction and, as shown in FIG. 5 (a), is equal to half a pitch of the bridge portions 20 in one of the corresponding rows. This cycle PA can be referred to as an effective pitch of the average luminous efficiency and is therefore referred to as "effective pitch" hereinafter.
On the other hand, as is well known to those skilled in the art, the luminescent phosphor dots 21 when excited by electron beams emit light and are successively excited sequentially from left to right on the screen according to the sweeping motion of the electron beams in a direction substantially perpendicular to the Y-axis direction, leaving scanning lines that are equally spaced from each other over the phosphor deposited screen 16 to form a field, each field or two interlaced fields comprising one frame of television pictures that is reproduced on the screen of the color cathode ray tube.
The density of excitation by electron bombardment on the phosphor deposited screen (that is, the electron beam density) TB (Y) has a cyclic characteristic in the Y-axis direction as shown in FIG. 6 and can be expressed by the following equation when transformed according to Fourier's series. ##EQU2## wherein PB represents the spacing between each neighboring scanning lines. In practice, the distribution L (Y) of luminance on the phosphor deposited screen in the Y-axis direction can be described as the product of the average luminous efficiency, expressed by the equation (1), multiplied by the excitation density expressed by the equation (2) and can therefore be expressed by the following equation. ##EQU3##
In equation (3), the second term represents the mosaic pattern of the luminous phosphor dots and the third term in the same equation (3) represents the pattern of components of the scanning lines. Also, the fourth term in the equation (3) can be rewritten as follows. ##EQU4##
While the first term in the equation (4) can be negligible because of the cyclic function smaller in pitch than any one of the mosaic pattern of the luminescent phosphor dots and the pattern of the scanning lines, the second term in the same equation (4) is the cyclic function which may give a very large spatial cycle (pitch) and would result in a noticeable pattern of large fringes which is objectionable to look at. This pattern of large fringes is referred to as Moire pattern and, therefore, the second term in the equation (4) is referred to as Moire term. When this Moire term is taken out, it can be expressed as follows. EQU .SIGMA..SIGMA.(AmBn/2).multidot.cos2.pi.Y(m/PA-n/PB) =.SIGMA..SIGMA.(AmBn/2).multidot.cos2.pi.Y[1/{PAPB/(mPB-nPA)}]
That is to say, the amplitude of change in intensity of light is expressed by (AmBn/2) and the pitch is expressed by [PAPB/(mPB-nPA)]. This pattern varies depending on respective specific values for m and n and, when (m, n) is fixed, one of them can be fixed. Moire pattern corresponding to a certain combination of m and n is referred to as Moire pattern in a mode (m, n).
One of the conditions in which Moire pattern becomes conspicuous on the screen of the color cathode ray tube occurs when this pitch becomes large or the value (mPB-nPA) takes a small value. In other words, it must satisfy the condition in which mPB becomes substantially equal to nPA.
On the other hand, it is usual that the respective values of Am and Bn are greater for m and n each being within the range of 1 to 3 than for m and n each being the value of 4 or greater.
Accordingly, generally in the color cathode ray tube, considerations have been paid that, for a given spacing PB between each neighboring scanning lines, the value of PA, that is, the spacing in the Y-axis direction of the phosphor deposited screen, and the corresponding pitch between each neighboring apertures in the shadow mask are so selected as to prevent one of mPB and nPA from becoming equal to the other of mPB and nPA with respect to the particular values (1 to 3) of m and n, so that Moire pattern in any mode will not exhibit an increased pitch (spatial cycle).
However, when it comes to the color cathode ray tube for use as computer output devices, that is, display devices wherein the number of scanning lines varies from one make to another, the spacing PB between each neighboring scanning lines can not be considered generally fixed. In such display devices, in order to minimize the appearance of Moire pattern in any mode of combinations of m and n, each being within the range of 1 to 3, for any expected number of scanning lines, that is, for any spacing between each neighboring scanning lines, the pitch PA between each neighboring aperture in the shadow mask must be adjusted. If the pitch PA is chosen to be small, the result would be that the screen becomes dark. On the other hand, if the pitch PA is chosen to be great, the mosaic pattern becomes coarse (with the consequent that the second term of the equation (3) become a problem) to such an extent as to result in the deteriorated screen and also a difficulty in maintaining a sufficient strength of the shadow mask.
In an attempt to minimize the appearance of Moire pattern, Japanese Laid-open Patent Publication No. 56-168473, published Dec. 24, 1981, discloses a technique wherein the overlapping between the interlaced first and second fields forming one frame of the television picture is slightly displaced from the position where they ought to have overlapped with each other. It has however been found that the technique disclosed in the above mentioned Japanese publication cannot be applicable to color cathode ray tubes other than the type operable with the interlaced scanning system.
The paper written by J. P. Wittke and entitled "Moire Considerations in Shadow-Mask Picture Tubes", published in SID 87 DIGEST, p. 347-350, May 1987, describes a technique of changing the vertical pitch, that is, the spacing between each neighboring slot-shaped apertures in the shadow mask, for minimizing the appearance of Moire pattern. According to this paper, it is obvious that the production of a number of shadow masks each having a different vertical pitch in the pattern of the apertures of the shadow mask are required for the system which operates on a different number of scanning lines.